Understanding gypsum growth

Ordered assembly of nanobricks observed for first time

Gypsum is one of three crystalline phases of calcium sulfate, alongside bassanite and anhydrite. Taking the form CaSO4·2H2O, gypsum is one of the most common minerals on Earth, yet its formation from ions in solution remains uncertain. Now researchers have used the Small Angle Scattering and Diffraction beamline (I22) to follow the formation of gypsum in situ for the first time.

The team used Small and Wide Angle X-ray Scattering (SAXS and WAXS) to show that gypsum forms in a four-stage process, beginning with the formation of well-defined calcium sulfate nanobricks, which self-assemble to form a crystalline structure. Time resolved measurements allowed the researchers to follow its growth in real time. The results published in Nature Communications give a new insight into gypsum nucleation and growth, which could potentially be used to understand its formation in nature in the future.

Calcium sulfate is a well-known and commonplace mineral, with large deposits of both the gypsum (CaSO4·2H2O) and anhydrite (CaSO4) crystallinephases found naturally on Earth. A third phase, bassanite (CaSO4·0.5H2O), while of highly limited abundance in natural settings, has an important role in the construction industry, with 100 billion tones of it produced annually as plaster of Paris. Along with gypsum, bassanite has also recently been found as a natural deposit on Mars.

Despite its prevalence, the way in which gypsum forms from ions in solution remains a mystery. Gaining a better understanding of the fundamental nucleation and growth of the mineral is key if scientists want to control its formation. Ultimately this could lead to more energy efficient routes of making bassanite, a process which is currently very energy intensive.

Previous studies have used various Transmission Electron Microscopy approaches to follow gypsum’s growth, however this technique requires samples to be frozen or dried and studied under a vacuum, each which may introduce unintended artefacts. Mindful of this problem, the pan-European team of researchers from institutes in the UK, Germany, Spain, Belgium, France, and Denmark, used the facilities at Diamond to develop a method that was truly in situ, meaning they followed the nucleation and crystallisation in solution and in real time.

Staged mineral growth The team used the I22 beamline to follow the formation of gypsum in solution by SAXS and WAXS and found that its growth occurs via the formation of calcium sulfate cores in a four-step process (Fig. 1).

“We found it doesn’t form directly from solution but instead is formed via calcium sulfate nanobricks which are the building blocks of gypsum,” said the lead author Dr Tomasz Stawski, from the German Research Centre for Geosciences. “We’ve called them nanobricks because the final structure is like a mosaic or wall.”

The four-step process begins with the formation of the bricks, which are well dispersed throughout the water matrix. In the second step the bricks move slowly closer to each other, beginning to form unordered domains and creating a disordered mosaic. Only in step three do the nanobricks start to self-assemble into a wall like structure, with them crystallising into gypsum in the fourth and final step.

Well-defined bricks I22 was key to following the mineralisation in situ, offering the team a way of studying the reaction using both SAXS and WAXS. Data was taken on average every 15 seconds in order to study nucleation and growth in real time using the flow-through reactor kit (Fig. 2). While the WAXS data allowed the team to interrogate the final crystalline structure formed in step four, for this study the SAXS data was especially important.

Fig. 2: In situ studies to observe the nucleation and growth of gypsum were performed on the flow-through reactor kit on I22.

“Self assembly is a fairly well known approach but it’s difficult to see because it happens at length scale which can only be probed by SAXS,” said co-author Professor Liane G. Benning from the University of Leeds and the German Research Centre for Geosciences. The SAXS technique allowed the team to follow every step of the reaction while it was happening in solution, rather than quenched and under vacuum, as in previous studies.

The SAXS data showed that the nanobricks were well defined, each being 3 nm long and 0.5 nm in cross-section. While the researchers were unable to determine whether bassanite was an intermediate in the process they were able to clearly show that gypsum formed via an aggregation of anhydrous cores before crystallisation.

Beamline support With help from the team at I22 the researchers developed a time-resolved approach to study environmental materials in situ that could be used for other mineral nucleation growth studies. The beamline team were also able to assist the team in writing their proposals, and discussing suitable sample environments in advance.

“The I22 team have been really helpful every time we’ve been there, and have allowed us to do the best experiments we can" said Prof Benning.

The researchers are now continuing their work on a different beamline – Extreme Conditions (I15) – where they are using Pair Distribution Function studies to discover the nature of the nanobricks.